In a modern sawmill operation, lumber is sawn into cants that have flat horizontal top and bottom surfaces and rough longitudinal side surfaces. The cants are processed through an edger to break them into boards of dimensioned lumber. Conventionally, the rough sawn boards are 1 or 2 inches thick, 2, 4, 6, 8, 10 or 12 inches wide and up to 20 feet long. From the edger, the boards are transferred onto an inclined transfer deck referred to as an unscrambler. As the boards advance along the unscrambler, they fall into a single tier of transversely oriented parallel boards with the longitudinal side surfaces of adjacent boards contacting each other. Typically, boards of varying thicknesses, widths and lengths are mixed together as they are processed through the mill. Within the mixture, there are commonly a significant number of boards that are twisted, warped, curved or otherwise irregularly shaped.
The next step in the milling process is to optical scan the boards. An optical scanner collects measurement data from each board passing through it and relays that information to a computer to create a morphological profile for use in further processing. Once the boards have been scanned, they are cut to length, and then sorted according to their dimensions.
The optical scanner can only take readings from a board which is spaced from adjacent boards. Accordingly, the boards advancing from the unscrambler must be singulated before passing through the scanner. Even if the boards are not optically scanned, they must be singulated before they are cut to length at the saw box. The most common method of singulating boards is to load them individually onto a lugged chain transfer with one board being placed between each pair of adjacent spaced lugs.
A number of devices that function to load lumber boards onto a lugged chain transfer are known. These devices, commonly referred to by those skilled in the art as either lug loaders or singulators, generally transfer the boards from an infeed transfer deck, on which the boards are positioned in parallel contacting relation, to an outfeed transfer deck, on which the boards are positioned in parallel spaced relation, for subsequent transfer onto a lugged chain transfer.
A number of singulators are known. In the apparatus disclosed in U.S. Pat. No. 3,031,061 issued to Rambo et al. on Apr. 24, 1962, lumber is fed from a driven conveyor belt onto a cam wheel having two cam segments with curved cam surfaces. Rotation of the cam wheel pushes the leading piece of lumber over a stationary stop and the pushing or pressing of the following pieces of lumber, together with the forward movement of the cam, causes the lumber to advance in a spaced manner according to the circumferential distance between the cam segments on the cam wheel. The singulator disclosed by Rambo et al. is not suitable for current sawmill or planer mill operations because it can only operate at low speeds. In addition, it cannot accommodate twisted, warped or curved pieces of lumber.
In U.S. Pat. No. 5,813,512 (Andersson) issued Sep. 29, 1998, a singulator having discs with cam surfaces is disclosed. Pieces of lumber are fed towards the discs on an infeed conveyor. The cam surfaces grippingly engage the leading piece of lumber and together with an upper feeder belt, lift and carry the piece of lumber forward onto an outfeed conveyor. The cam surfaces may have teeth to assist in gripping the underside of the lumber. The speed at which this singulator can operate is limited by the fact that each piece of lumber must be gripped by a cam surface. If the operating speed of the singulator is increased to the desired rate, it has a tendency to double load by depositing two pieces of lumber onto a single lug. Double loading substantially reduces the efficiency of the singulating operation because if more than one piece of lumber is placed between lugs, the optical scanner, unable to scan the pieces, transmits a “reject” signal to the sorter and the piece must be re-fed through the mill or planer. Increasing the speed also has a tendency to cause the gripping surface of the cam wheel to slide across the bottom surface of the lumber instead of engaging it. This not only results in a failure to advance the lumber piece onto the outfeed conveyor but also damages the bottom surfaces of the pieces.
A further drawback of the singulator disclosed in the '512 patent is that it can only accommodate thickness variations of up to ¼ inch without having to adjust the spacing between the cam surfaces and the feeder belts which operate above them. Moreover, the singulator does not adequately accommodate pieces of lumber that are twisted, bowed, warped or otherwise irregularly shaped because such pieces do not properly contact the cam surfaces and are consequently improperly gripped by them. This skipped loading affects the fill rate of the singulator, and as the speed of the singulator is increased, the fill rate drops.
Another singulator is disclosed in U.S. Pat. No. 5,921,376 (Michell et al.) issued Jul. 13, 1999. This singulator has a disc with heel and hook assemblies radially spaced thereon. The assemblies grasp the pieces of lumber along their lengths and individually transfer the pieces from the infeed transfer chain to the outfeed transfer chain. Because each piece of lumber is individually grasped by a heel and hook assembly, this singulator cannot attain the desired operating speed. In addition, it has a large number of moving parts which tends to result in increased maintenance requirements. Moreover, the heel and hook assemblies do not sufficiently accommodate pieces of lumber of different dimensions or pieces which are twisted, bowed, warped or otherwise irregular. Another drawback of this singulator is that engaging the lumber in the manner of the heel and hook assemblies tends to make marks on the lumber. These marks cause the end products to be downgraded which, in turn, results in decreased revenues for the mill operator.
Thus, there is a need for a singulator that can operate at high speed yet maintain a high fill rate and not double load or damage the lumber. In addition, there is a need for such a singulator to be able to accommodate lumber of widely varying dimensions and lumber that is twisted, bowed, warped or otherwise irregular.